Former British prime minister Tony Blair will appear before an inquiry into the Iraq War for a second public grilling on Friday to clarify his earlier evidence detailing his reasons for joining the invasion.

Blair, who sent 45,000 British troops as part of the US-led invasion in 2003, told the London inquiry in his first appearance that Saddam Hussein had been a threat to the world who had to be removed or disarmed.

He also said he had no regrets about the military action, a comment that angered some of the relatives of the 179 British soldiers killed in Iraq.

Alistair Campbell, Blair's former communications chief and one of his closest advisers until he resigned in late 2003, said people still felt raw about the war.

'Some people who actually really liked Tony Blair when he became prime minister they will never forgive him for Iraq,' he told Sky News.

The decision to go to war was one of the most controversial episodes of Blair's 10-year premiership which ended in 2007, leading to massive protests and accusations he had deliberately misled the public over the reasons for the invasion.

Blair denied such claims and rejected suggestions he had promised US President George W. Bush he would support military action in 2002, months before attempts to secure explicit UN backing had foundered.

He also said the war was legal based on advice he had been given from the government's then top lawyer.

The inquiry, which began in Nov. 2009 and is headed by former civil servant John Chilcot, was set up by Blair's successor Gordon Brown to learn lessons from the conflict and is not designed to assign guilt or blame to any individual.

Blair's six-hour appearance in January last year has been the highlight of the probe which has heard from a host of senior military and political figures from Britain and abroad. He is one of a small number of witnesses to have been recalled.

Other witnesses have offered evidence which appeared to conflict with some details Blair gave, and one former minister said he had lied to his cabinet before the invasion.

'As we begin to write our report, there are a few remaining areas where we need to clarify exactly what happened,' committee chairman John Chilcot said on Tuesday.

Hostility over Iraq continues to dog Blair, 57, now an envoy for the Quartet of Middle East peacemakers -- the United States, Russia, the EU and the United Nations.

Opponents of the war said they would be demonstrating outside the inquiry's venue near parliament in central London.

'Evidence has now emerged at Chilcot showing Blair lied to public and parliament about the legality of an attack on Iraq,' said Chris Nineham from Stop the War Campaign.-Reuters